I have been assuming it was caused by the shedding of spike proteins.
You are right that is the more likely explanation. I am not actually making this up... Every time we have been in close contact with people bragging about their vaccination status we have had froggy voices, scratchy throats, headaches, and been feeling under the weather the next day. It is just plain weird, but my wife said a week ago that anyone who starts bragging about their vaccination status is going to be asked to leave.
https://chemicalviolence.com/2021-05-26-vaccine-spike-proteins-are-spreading-more-disease.html